The ras oncogenes are frequently activated by point mutations or overexpression in human tumors (Barbacid, M. Eur. J. Clin. Invest. 1990, 20(3):225-235; Bos, J. L. Cancer Res. 1989, 49:4682-4689). Approximately 30% of all human tumors are associated with ras mutations and up to 95% of human pancreatic cancers contain K-ras mutations (Barbacid, M. Eur. J. Clin. Invest. 1990, 20(3):225-235; Bos, J. L. Cancer Res. 1989, 49:4682-4689). Ras proteins are believed to mediate their oncogenic effect, in part, by deregulating the cell cycle machinery (Downward, J. Curr. Biol. 1997, 7:R258-R260; Ewen, M. E. Prog. Cell Cycle 2000 4:1-17; Kerkhoff, E. and U. R. Rapp Oncogene 1998, 17:1457-1462). For instance, activated Ras, acting through the Raf/Mek/Erk kinase pathway, was shown to increase cyclin D1 expression and to shorten the G1 phase of the cell cycle (Aktas, H. et al. Mol. Cell Biol. 1997, 17:3850-3857; Arber, N. et al. Oncogene 1996, 12:1903-1908; Filmus, J. et al Oncogene 1994 9:3627-3633; Hitomi, M. and D. W. Stacey Mol. Cell Biol. 1999 19:4623-4632; Liu, J. J. et al. Mol. Cell Biol. 1995, 15:3654-3663; Peeper, D. S. et al. Nature 1997, 386:177-181; Robles, A. I. et al. Genes Dev. 1998, 12:2469-2474; Winston, J. et al., J. Biol. Chem. 1996, 271:11253-11260).
U.S. Pat. No. 6,191,342 describes a transgenic mouse having a genome containing the H/K-ras 4B chimeric gene to form a mammary tumor, where the mouse was prepared using an expression vector producing H/K-Ras 4B chimeric protein and having a MMTV (mouse mammary tumor virus) promoter. The expressed protein contains the first 164 amino acids of the H-Ras followed by the last 24 amino acids of K-Ras 4B.
Such MMTV/v-Ha-ras transgenic mice (Sinn E. et al., Cell May 1987, 22:49(4):465-475), which routinely develop mammary tumors, are used for the study of tumorigenesis and for anti-cancer drug testing. However MMTV/v-Ha-ras female mice are unable to nurse their young and, therefore, in order to breed these mice, the newborn pups must be fostered to a surrogate nursing female mouse. Usually, this fostering must be accomplished within 8 to 10 hours after birth or the survival of these newborn pups is greatly compromised. Fostering imposes significant practical and technical obstacles to breeding these mice and, therefore, impacts a researcher's ability to obtain sufficient mice for study. The obstacles include the maintenance of a sufficient number of foster male and female breeders to ensure a steady supply of nursing mothers to be used as surrogates for the newborn transgenic pups. Thus, the expense of the required foster mice must be added to the expense of the project. Also, foster mice will often not nurse the newborn transgenic pups, especially if their own pups are more than a few days old, resulting in unpredictable, expensive and wasteful loss of animals that may potentially compromise experimental results.
U.S. Pat. No. 5,688,665 discloses an isolated protein having an apparent molecular weight of about 27 kD and capable of binding to and inhibiting the activation of a cyclin E-Cdk2 complex, which is termed p27KIP-1. U.S. Pat. No. 6,180,333 discloses methods of grading tumors and prognosticating survival rates of cancer patients by determination of p27 expression levels in tissue samples from tumors. Transgenic studies of the effect of p27 on tumorigenesis are hampered by the observation that female p27 knockout mice (Fero, M. L. et al. Cell May 1996, 85(5):733-744) of B6.129-cdKn1btm1MLF background (also termed “C57BL/6 and 129”) are sterile. Therefore, these mice inherently present husbandry challenges.
There is, therefore, a need for animal models of tumorigenesis that do not exhibit the aforementioned disadvantages and limitations. Such models would facilitate the husbandry and experimental use of transgenic models of tumorigenesis, thereby aiding in the development of treatments for tumors and the prevention of tumorigenesis. Such models would further facilitate the investigation and development of the treatment of tumorigenesis in animals having more than one transgenic gene, by eliminating husbandry problems associated with the crossing of animals that may be unable to nurse or are sterile.